1. Field of the Invention
This invention relates to mechanical devices that have a component in which large recoverable distortions are advantageous.
2. Description of the Related Art
Shape memory alloy materials (also termed SMA) are well known. One Common SMA material is TiNi (also known as nitinol), which is an alloy of nearly equal atomic content of the elements Ti and Ni. Such an SMA material will undergo a crystalline phase transformation from martensite to austenite when heated through the material s phase change temperature. When below that temperature the material can be plastically deformed from a “memory shape” responsive to stress. When heated through the transformation temperature, it reverts to the memory shape while exerting considerable force.
In the prior art many different useful devices employing SMA have been developed and commercialized. The typical SMAs used in the prior art devices are of polycrystalline form. Polycrystalline SMA exhibits both: 1) shape memory recovery (when cycled through the material's transformation temperature) and 2) superelasticity. The term superelasticity applies to an SMA material which, when above the transformation temperature (in the austenite crystalline phase), exhibits a strain recovery of several percent. This is in comparison to a strain recovery on the order of only about 0.5 percent for non-SMA metals and metal alloys.
Superelasticity results from stress-induced conversion from austenite to martensite as stress is increased beyond a critical level, and reversion from martensite to austenite as stress is reduced below a second (lower) critical level. These phenomena produce a pair of plateaus of constant stress in the plot of stress versus strain at a particular temperature. Single crystal superelasticity is characterized by an abrupt change in slope of the stress strain plot at a combination of stress, strain, and temperature characteristic of that particular alloy.
Shape memory copper-aluminum based alloys grown as single crystals have been experimentally made in laboratories, typically in combination with about 5 percent Ni, Fe, Co, or Mn. The most common such CuAl-based alloy is CuAlNi, which is used throughout this description as the primary example: others are CuAlFe, CuAlCo, and CuAlMn. Single crystal SMA materials when stressed have the property of enabling a shape memory strain recovery much greater than polycrystalline SMA, and superelastic shape recovery as great as 24 percent.